The present invention relates to an actuation circuit for a liquid crystal display including pixel elements that can be actuated to display a signal and are constructed in the form of a matrix M of columns and rows as shown in FIG. 1a.
Prior art liquid crystal displays include two substrates which are arranged at a defined distance from one another. Between the substrates there is a liquid crystal material. The substrates are selected in such a way that, depending on their use, one or both substrates are transparent. A transparent ground plane conductor is disposed on one of these substrates. The oppositely disposed substrate includes a matrix of individual electrode elements, called pixel electrodes. A semiconductor switch, preferably a thin-film transistor, is associated with each one of these pixel electrodes and is disposed on the substrate provided with these pixel electrodes. These transistor switches are made either of amorphous silicon or of polycrystalline silicon. Due to the lower processing temperatures, the amorphous silicon technology is preferable for consumer uses.
With reference to FIG. 1a the individual pixels 120 of the matrix M act as capacitors, with the liquid material serving as the dielectric substance. The application of a voltage to a pixel electrode controls the light transmittance of the liquid crystal material. The transformation or regrouping of the liquid crystals permits a display of text or graphic information which becomes visible on the device. Each pixel electrode has its own associated semiconductor switch 122 which can be turned on or off so that each individual pixel element can be controlled by signals supplied to its associated semiconductor switch.
Each semiconductor switching element or transistor 122 is disposed at the point of intersection between a column m and a row n of the matrix of pixel electrodes. The columns m are data lines, rows n are selection lines. The control gate electrode 124 of each transistor switch is connected with a selection line 126 and the source or drain electrode of the transistor switch is connected with a data line 128. The drain or source electrode is connected with the pixel electrode.
During a display operation, a line is written in that a signal level is constantly applied to each one of the data lines. The selection lines are continuously activated so that the voltages occurring in the data lines are applied by their respective semiconductor switching elements to the pixel electrodes. During playback, for example of picture information, the voltages representing, for example, gray values are applied to data lines and the selection lines are activated for the respective line to be written. After the necessary, short charging time for the liquid capacitor, another selection line is activated and the data voltages associated with this row are sequentially applied to the pixel columns. For television applications, this writing occurs from the top to the bottom in, for example, 1/25 or 1/50 of a second. Thus a complete picture is displayed on the screen in this time period.
In addition to the capacitance of the liquid crystal capacitor, there are parasitic capacitances which undesirably influence the signal transmission from the data lines to the liquid crystal capacitors. On the one hand, there are parasitic capacitances between the data lines and the pixel electrodes, and on the other hand, there are parasitic gate/source and gate/drain capacitances in the switching element itself. During operation of such a display, the voltage across one pixel is set during its purely addressing phase. The semiconductor switch is then blocked, and the voltage should remain constant until the line is rewritten during the renewed build-up of a picture.
A minimum charging time is required to charge the liquid crystal capacitor to the voltage value predetermined by the information in the data line and to charge the parasitic capacitances. This charging time cannot be reduced at will because of the low mobility of the amorphous silicon employed.
European Patent Application No. Al-0,298,255 discloses an actuation circuit for a liquid crystal display including an active matrix display and including signal driver circuits for the columns and rows. The column actuation circuit includes shift registers as well as two sample-and-hold circuits for each column. The data of a row are written sequentially into the first sample-and-hold circuit. Once a row has been written, its entire contents are transferred to the second sample-and-hold circuit which actuates the liquid crystal display elements. In this process, it is necessary to take care that the voltage of the previous columns is kept constant over a long period of time until the sample-and-hold circuit of the last column has been charged. Moreover, a separate connecting line is required for each column.